powerpc: T4160RDB: Separate from T4240RDB in Kconfig

Use TARGET_T4160RDB to simplify Kconfig options.

Signed-off-by: York Sun <york.sun@nxp.com>
diff --git a/board/freescale/t4rdb/Kconfig b/board/freescale/t4rdb/Kconfig
index d93e453..67832da 100644
--- a/board/freescale/t4rdb/Kconfig
+++ b/board/freescale/t4rdb/Kconfig
@@ -1,4 +1,4 @@
-if TARGET_T4240RDB
+if TARGET_T4160RDB || TARGET_T4240RDB
 
 config SYS_BOARD
 	default "t4rdb"